Driving circuit for piezoelectric transformer, cold-cathode tube light-emitting apparatus, liquid crystal panel and device with built-in liquid crystal panel

ABSTRACT

A driving circuit for a piezoelectric transformer is provided, which ensures lighting of all the cold-cathode tubes connected to the piezoelectric transformer, and reduces the difference in brightness between the cathode tubes during steady lighting, thereby enhancing reliability and performance. A plurality of cold-cathode tubes connected to a secondary side of the piezoelectric transformer, and a cold-cathode tube output detector circuit connected in series to a plurality of cold-cathode tubes, for detecting an output state of the respective cold-cathode tubes are provided, and the driving of the piezoelectric transformer is controlled based on a detection signal from the cold-cathode tube output detector circuit. Because of this, the piezoelectric transformer performs the same operation as that with respect to one cold-cathode tube.

This application is a divisional of U.S. application Ser. No.10/306,516, filed Nov. 27, 2002, pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a driving circuit for a piezoelectrictransformer, a cold-cathode tube light-emitting apparatus using acold-cathode tube as a load of a piezoelectric transformer, a liquidcrystal panel in which the cold-cathode tube light-emitting apparatus isbuilt so as to control the brightness, and devices with the liquidcrystal panel built therein.

2. Description of the Related Art

Hereinafter, a conventional piezoelectric transformer and drivingcircuit for the piezoelectric transformer will be described withreference to the drawings.

The piezoelectric transformer has a configuration in which a primary(input) side electrode and a secondary (output) side electrode areformed on a piezoelectric body, an AC voltage in the vicinity of aresonance frequency of the piezoelectric transformer is applied to theprimary side electrode to vibrate the piezoelectric transformermechanically, and the mechanical vibration is converted by apiezoelectric effect so as to be output from the secondary sideelectrode as a high voltage. The piezoelectric transformer can berendered smaller and thinner, compared with an electromagnetictransformer, so that a high conversion efficiency can be realized.

FIG. 14 shows a perspective view of a conventional piezoelectrictransformer 101 with one output on the secondary side. In FIG. 14, thepiezoelectric transformer 101 has the following configuration: primary(input) side electrodes 102 and 103 are formed over substantially halfof the principal planes of a rectangular plate 105 made of apiezoelectric ceramic material (e.g., lead zirconate titanate (PZT)) soas to be opposed in the thickness direction, and a secondary (output)side electrode 104 is formed on one end face of the rectangular plate105 in the length direction. The rectangular plate 105 is previouslypolarized in the thickness direction using the electrodes 102 and 103,and is previously polarized in the length direction using the electrodes102, 103 and the electrode 104. When an AC voltage in the vicinity ofthe resonance frequency of vibration that expands and contracts in thelength direction of the piezoelectric transformer 101 is applied betweenthe primary side electrodes 102 and 103, the piezoelectric transformer101 excites mechanical vibration that expands and contracts in thelength direction. The mechanical vibration is converted into a voltageby a piezoelectric effect. The voltage thus obtained is output from theelectrode 104 in accordance with an impedance ratio between the primaryside electrodes and the secondary side electrode.

FIG. 15 shows a perspective view of a piezoelectric transformer 111 withtwo outputs on the secondary side. In FIG. 15, the piezoelectrictransformer 111 has the following configuration: primary side electrodes112 and 113 are formed substantially at the center of a rectangularplate 116 (made of a piezoelectric ceramic material) in the lengthdirection so as to be opposed in the thickness direction, a secondaryside electrode 114 is formed on one end face of the piezoelectrictransformer 111 in the length direction, and another secondary sideelectrode 115 is formed on the other end face thereof in the lengthdirection. The rectangular plate 116 is previously polarized in thethickness direction using the electrodes 112 and 113, and is previouslypolarized in the length direction using the electrodes 112, 113 and theelectrodes 114 and 115. When an AC voltage in the vicinity of theresonance frequency of vibration that expands and contrasts in thelength direction of the piezoelectric transformer 111 is applied betweenthe electrodes 112 and 113, the piezoelectric transformer 111 excitesmechanical vibration that expands and contracts in the length direction.The mechanical vibration is converted into a voltage by a piezoelectriceffect. The voltage thus obtained is output from the electrodes 114 and115 in accordance with an impedance ratio between the primary sideelectrodes and the secondary side electrodes.

Generally, in the piezoelectric transformer, due to the impedance of aload connected to the secondary side, a voltage step-up ratio, whichrepresents a ratio between a voltage input to the primary side and avoltage output from the secondary side, is varied. Furthermore, adriving efficiency represented by the electric power output from thesecondary side with respect to the electric power input to the primaryside is varied similarly. Therefore, the driving frequency also isvaried, which enables the maximum voltage step-up ratio and drivingefficiency to be obtained.

For example, in the case of using a cold-cathode tube as a load of thepiezoelectric transformer, the cold-cathode tube generally exhibits ahigh impedance equal to or more than hundreds of MΩ until it lights up,and the impedance decreases rapidly to a range between hundreds of Ω andtens of Ω after it lights up. Furthermore, a voltage equal to or morethan several kV is required for allowing the cold-cathode tube to lightup, and even during steady lighting, a voltage from hundreds of V toseveral kV is required. In order to allow the cold-cathode tube to lightup efficiently by using the piezoelectric transformer, it is required tochange the frequency and the level of an AC voltage applied to theprimary side of the piezoelectric transformer between a period beforethe commencement of lighting and a period after lighting.

As a prior art for realizing the above, a cold-cathode tube drivingapparatus described in JP 6(1994)-167694 A is known. FIG. 16 shows ablock diagram of a driving apparatus disclosed in this publication.

In FIG. 16, an output signal from a free-running multivibrator 206 isamplified by a current amplifier 207, and a voltage further isstepped-up by a wire-wound transformer 208, if required, to be appliedto the primary side of a piezoelectric transformer 201. A cold-cathodetube 202 is connected as a load to a secondary side output of thepiezoelectric transformer 201. A current flowing through thecold-cathode tube 202 is detected by a load current detector circuit209. The detected current is converted to a voltage. The voltage thusobtained is input to one input terminal of an integrator circuit 204through an AC voltage rectifier circuit 210, and a signal from avariable voltage apparatus 203 is supplied to the other input terminalof the integrator circuit 204. In this manner, an oscillation frequencyof the free-running multivibrator 206 is controlled by the integratorcircuit 204 through a voltage level shift circuit 205.

A voltage applied to the piezoelectric transformer 201 is set by thevariable voltage apparatus 203, the voltage level shift circuit 205, andthe like, and the driving frequency of the piezoelectric transformer 201is swept, whereby the cold-cathode tube 202 that is a load of thepiezoelectric transformer 201 is allowed to light up. After lighting,the driving frequency of the piezoelectric transformer 201 again isswept. Furthermore, a voltage applied to the piezoelectric transformer201 is controlled by the variable voltage apparatus 203, the voltagelevel shift circuit 205, and the like in accordance with the level of acurrent detected by the load current detector circuit 209 and the like,whereby the light-emission brightness of the cold-cathode tube 202 isadjusted.

The case where a plurality of loads such as a cold-cathode tube areconnected to the piezoelectric transformer is disclosed by, for example,JP 8(1996)-45679 A. In this disclosure, as shown in FIG. 17,cold-cathode tubes 120 and 121 are connected in series to the secondaryside electrode 104 of the piezoelectric transformer 101 with one outputon the secondary side shown in FIG. 14. In this case, both ends of onecold-cathode tube need to be supplied with a voltage of several kV untilthe commencement of lighting and a voltage of hundreds of V duringsteady lighting. Therefore, the piezoelectric transformer 101 isrequired to output a high voltage based on the number of cold-cathodetubes to be connected.

Therefore, it is required to configure the connection portion and wiringbetween the piezoelectric transformer 101 and the cold-cathode tube 120,and a circuit board so that they withstand a high voltage. Furthermore,regarding the mounting of circuit components including the piezoelectrictransformer on the circuit board, it is required to enlarge the distancebetween the respective components so as to avoid an insulation breakdowndue to a high voltage and enhance safety. Therefore, there is a limit tothe enhancement of a mounting density. Even if the piezoelectrictransformer and the circuit components are miniaturized, a systemincluding the circuit board cannot be miniaturized and its space cannotbe reduced.

Furthermore, there is another conventional example in which a pluralityof cold-cathode tubes are connected to the piezoelectric transformerwith one output on the secondary side shown in FIG. 14. Morespecifically, as shown in FIG. 18, the cold-cathode tubes 120 and 121are connected in parallel to the secondary side electrode 104 of thepiezoelectric transformer 101. In this case, due to the variation ofimpedances of the cold-cathode tubes 120 and 121, a lightingcommencement time is varied therebetween. For example, when thecold-cathode tube 120 starts lighting up first, the impedance of thecold-cathode tube 120 rapidly is decreased. Because of this, the voltagestep-up ratio of the piezoelectric transformer 101 is decreased, and thecold-cathode tube 121 other than the cold-cathode tube 120 that hasstarted lighting up first cannot keep a voltage level at which lightingcan start. As a result, only one cold-cathode tube lights up.

Furthermore, JP 8(1996)-45679 A discloses another conventional examplein which a plurality of cold-cathode tubes are connected to thepiezoelectric transformer 111 with two outputs on the secondary sideshown in FIG. 15. In this disclosure, as shown in FIG. 19, thecold-cathode tubes 120 and 121 are connected in series to the secondaryside electrodes 114 and 115 of the piezoelectric transformer 111. Unlikethe method shown in FIG. 18, the following state can be avoided: due tothe variation of impedances of the cold-cathode tubes 120 and 121, onlyone cold-cathode tube lights up while the other does not.

Since the connection portion between the cold-cathode tubes 120 and 121is connected electrically to one primary side electrode 113,non-lighting can be avoided. However, the difference in brightness iscaused by the difference in impedance between two cold-cathode tubesduring steady lighting. In addition, the piezoelectric transformer 111needs to perform an unstable operation of continuously supplyingdifferent electric powers from two secondary side electrodes 114 and115.

SUMMARY OF THE INVENTION

Therefore, with the foregoing in mind, it is an object of the presentinvention to provide a driving circuit for a piezoelectric transformerthat ensures lighting of all the cold-cathode tubes when a plurality ofcold-cathode tubes are connected to a piezoelectric transformer andreduces the difference in brightness between respective cold-cathodetubes to an unrecognizable level during steady lighting, therebyenhancing reliability and performance; a cold-cathode tubelight-emitting apparatus using the driving circuit for a piezoelectrictransformer; a liquid crystal panel in which the cold-cathode tubelight-emitting apparatus is built so as to control the brightness; anddevices with the liquid crystal panel built therein.

In order to achieve the above-mentioned object, a first driving circuitof the present invention for a piezoelectric transformer, which includesa primary side electrode and a secondary side electrode formed on apiezoelectric body and converting an AC voltage input from the primaryside electrode to output the converted AC voltage from the secondaryside electrode, includes: a plurality of loads connected to a secondaryside of the piezoelectric transformer; and a load state detector portionconnected in series to the plurality of loads, for detecting an outputstate of each load, wherein driving of the piezoelectric transformer iscontrolled based on a detection signal from the load state detectorportion.

According to the above-mentioned configuration, in the case wherecold-cathode tubes are used as loads of the piezoelectric transformer,even if a plurality of cold-cathode tubes have different impedances, thedifference in brightness between the respective cold-cathode tubes issmall during steady lighting, and non-lighting of the cold-cathode tubesat the commencement of lighting is unlikely to occur.

In order to achieve the above-mentioned object, a second driving circuitof the present invention for a piezoelectric transformer including aprimary side electrode and a secondary side electrode formed on apiezoelectric body and converting an AC voltage input from the primaryside electrode to output the converted AC voltage from the secondaryside electrode includes: a plurality of loads connected to a secondaryside of the piezoelectric transformer; and a load state detector portionconnected in series to the plurality of loads, for electricallyseparating and detecting an output state of each load, wherein drivingof the piezoelectric transformer is controlled based on a detectionsignal from the load state detector portion.

According to the above-mentioned configuration, in the case wherecold-cathode tubes are used as loads of the piezoelectric transformer,in addition to the effect of the first driving circuit for apiezoelectric transformer, a ratio at which a current flowing throughthe cold-cathode tubes and a detection signal influence each other canbe suppressed. Therefore, noise resistance of both a current flowingthrough the cold-cathode tubes and a detection signal can be enhanced.

In order to achieve the above-mentioned object, a third driving circuitof the present invention for a piezoelectric transformer, which includesa primary side electrode and a secondary side electrode formed on apiezoelectric body and converting an AC voltage input from the primaryside electrode to output the converted AC voltage from the secondaryside electrode, includes: a plurality of loads connected to a secondaryside of the piezoelectric transformer; a load state detector portionconnected in series to the plurality of loads, for detecting an outputstate of each load; and a frequency selection portion for selectivelyoutputting only a frequency component in a vicinity of a drivingfrequency of the piezoelectric transformer in a detection signal fromthe load state detector portion, wherein driving of the piezoelectrictransformer is controlled based on a signal from the frequency selectionportion.

According to the above-mentioned configuration, in the case wherecold-cathode tubes are used as loads of the piezoelectric transformer,in addition to the effect of the first driving circuit for apiezoelectric transformer, an unnecessary harmonic component can beremoved, and the driving control of the piezoelectric transformer andthe brightness control of the cold-cathode tubes can be performed with ahigher precision.

In order to achieve the above-mentioned object, a cold-cathode tubelight-emitting apparatus of the present invention includes: apiezoelectric transformer including a primary side electrode and asecondary side electrode formed on a piezoelectric body and convertingan AC voltage input from the primary side electrode to output theconverted AC voltage from the secondary side electrode; a plurality ofcold-cathode tubes connected to a secondary side of the piezoelectrictransformer; a cold-cathode tube output detector portion connected inseries to the plurality of cold-cathode tubes, for electricallyseparating and detecting a current flowing through the respectivecold-cathode tubes; and a control portion for controlling light-emissionof the plurality of cold-cathode tubes based on a detection signal fromthe cold-cathode tube output detector portion.

In order to achieve the above-mentioned object, a liquid crystal panelof the present invention has its brightness controlled by a built-incold-cathode tube light-emitting apparatus, and the cold-cathode tubelight-emitting apparatus includes: a piezoelectric transformer includinga primary side electrode and a secondary side electrode formed on apiezoelectric body and converting an AC voltage input from the primaryside electrode to output the converted AC voltage from the secondaryside electrode; a plurality of cold-cathode tubes connected to asecondary side of the piezoelectric transformer; a cold-cathode tubeoutput detector portion connected in series to the plurality ofcold-cathode tubes, for electrically separating and detecting a currentflowing through the respective cold-cathode tubes; and a control portionfor controlling light-emission of the plurality of cold-cathode tubesbased on a detection signal from the cold-cathode tube output detectorportion.

According to the above-mentioned configuration, an output voltage of apiezoelectric transformer is varied in accordance with a loadfluctuation at the commencement of lighting and during lighting of thecold-cathode tubes. Therefore, an adverse effect on another circuitsystem mounted on a liquid crystal panel can be eliminated. Furthermore,an output voltage to the cold-cathode tubes has a substantially sinewave, so that an unnecessary frequency component that does notcontribute to lighting of the cold-cathode tubes also can be reduced.Furthermore, the piezoelectric transformer can be mounted even in anarrow place such as an edge of a liquid crystal panel, which leads tominiaturization and reduced weight of the liquid crystal panel.Furthermore, a lighting defect of the cold-cathode tubes is decreased,and the piezoelectric transformer can be driven stably, which leads tohigh reliability and high performance of the liquid crystal panelitself.

In order to achieve the above-mentioned object, a device with a built-inliquid crystal panel of the present invention incorporates a liquidcrystal panel whose brightness is controlled by a built-in cold-cathodetube light-emitting apparatus. The cold-cathode tube light-emittingapparatus includes: a piezoelectric transformer including a primary sideelectrode and a secondary side electrode formed on a piezoelectric body,for converting an AC voltage input from the primary side electrode tooutput the converted AC voltage from the secondary side electrode; aplurality of cold-cathode tubes connected to a secondary side of thepiezoelectric transformer; a cold-cathode tube output detector portionconnected in series to the plurality of cold-cathode tubes, forelectrically separating and detecting a current flowing through therespective cold-cathode tubes; and a control portion for controllinglight-emission of the plurality of cold-cathode tubes based on adetection signal from the cold-cathode tube output detector portion.

According to the above-mentioned configuration, even when the number ofcold-cathode tubes is small, high brightness of a liquid crystal screencan be realized. In addition, by using a piezoelectric transformerinstead of an electromagnetic transformer in order to allow thecold-cathode tubes to light up, the generation of electromagnetic noisecan be suppressed, and noise to a device and an adverse effect due tocross modulation can be eliminated.

In the first to third driving circuits for a piezoelectric transformer,cold-cathode tube light-emitting apparatus, liquid crystal panel, anddevices with a built-in liquid crystal panel according to the presentinvention, the piezoelectric body has a rectangular shape and thesecondary side electrode of the piezoelectric transformer is provided oneach end face of the piezoelectric body in a longitudinal direction, andpolarization directions of the piezoelectric body in a vicinity of thesecondary side electrodes are the same in the longitudinal direction.

According to the above-mentioned configuration, AC voltages havingsubstantially the same amplitude and a phase difference of 180° can beoutput from two secondary side electrodes.

Furthermore, in the first to third driving circuits for a piezoelectrictransformer, cold-cathode tube light-emitting apparatus, liquid crystalpanel, and devices with a built-in liquid crystal panel according to thepresent invention, in the case where cold-cathode tubes are used asloads of the piezoelectric transformer, it is preferable that the numberof the cold-cathode tubes is 2n (n is an integer of 1 or more).

According to the above-mentioned configuration, a voltage applied to thecold-cathode tubes becomes substantially zero at a connection portion ofthe cold-cathode tubes instead of the inside thereof. Therefore, a darkportion at which the brightness is decreased at all times can beeliminated.

Furthermore, in the second driving circuit for a piezoelectrictransformer, cold-cathode tube light-emitting apparatus, liquid crystalpanel, and devices with a built-in liquid crystal panel according to thepresent invention, it is preferable that the load state detector portionincludes an optical isolator composed of a light-emitting diode and aphototransistor.

According to the above-mentioned configuration, in the case wherecold-cathode tubes are used as loads of the piezoelectric transformer, acurrent flowing through the cold-cathode tubes is separated electricallyfrom a detection signal without using an inductive element. Therefore,even when the difference in impedance between a plurality ofcold-cathode tubes is large, there is no noise generation factor, andnoise resistance of both a current flowing through the cold-cathodetubes and a detection signal can be enhanced further.

Furthermore, in the cold-cathode tube light-emitting apparatus, liquidcrystal panel, and devices with a built-in liquid crystal panelaccording to the present invention, it is preferable that thepiezoelectric transformer and the control portion are mounted on a firstsubstrate placed in proximity to one electrode of each of the pluralityof cold-cathode tubes, and the cold-cathode tube output detector portionis mounted on a second substrate placed in proximity to the otherelectrode of each of the plurality of cold-cathode tubes.

According to the above-mentioned configuration, a portion that dealswith a relatively large electric power including the piezoelectrictransformer is separated from a portion that deals with a small electricpower including the cold-cathode tube output detector portion by usingseparate substrates. Because of this, the ability of the cold-cathodetube output detector portion to detect a current flowing through thecold-cathode tubes can be enhanced, and noise (in particular, noise dueto the proximity to a ground line) can be uppressed from being mixed ina detection signal output from the cold-cathode tube output detectorportion.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an exemplary configuration of adriving circuit for a piezoelectric transformer of Embodiment 1according to the present invention.

FIG. 2 is a block diagram showing the exemplary configuration of thedriving circuit for a piezoelectric transformer shown in FIG. 1, and anexemplary internal configuration of a cold-cathode tube output detectorcircuit 20.

FIG. 3 is a block diagram showing an exemplary configuration of adriving circuit for a piezoelectric transformer of Embodiment 2according to the present invention and an exemplary internalconfiguration of the cold-cathode tube output detector circuit 20.

FIG. 4 is a block diagram showing an exemplary configuration of adriving circuit for a piezoelectric transformer of Embodiment 3according to the present invention, and an exemplary internalconfiguration of the cold-cathode tube output detector circuit 20.

FIG. 5 is a schematic view showing an internal configuration of a liquidcrystal panel of Embodiment 4 according to the present invention.

FIG. 6 is a schematic view showing a configuration of a mobile phone ofEmbodiment 5 according to the present invention.

FIG. 7 is a block diagram showing an exemplary configuration of adriving circuit for a piezoelectric transformer of Embodiment 6according to the present invention, and an exemplary internalconfiguration of the cold-cathode tube output detector circuit 20.

FIG. 8 is a block diagram showing an exemplary configuration of adriving circuit for a piezoelectric transformer of Embodiment 7according to the present invention, and an exemplary internalconfiguration of the cold-cathode tube output detector circuit 20.

FIG. 9 is a block diagram showing an exemplary configuration of adriving circuit for a piezoelectric transformer of Embodiment 8according to the present invention.

FIG. 10 is a block diagram showing the exemplary configuration of adriving circuit for a piezoelectric transformer shown in FIG. 9, and anexemplary internal configuration of the cold-cathode tube outputdetector circuit 20.

FIG. 11 is a block diagram showing an exemplary configuration of adriving circuit for a piezoelectric transformer of Embodiment 9according to the present invention.

FIG. 12 is a block diagram showing the exemplary configuration of adriving circuit for a piezoelectric transformer shown in FIG. 11, and anexemplary internal configuration of the cold-cathode tube outputdetector circuit 20.

FIG. 13 is a schematic view showing an internal configuration of aliquid crystal panel of Embodiment 10 according to the presentinvention.

FIG. 14 is a perspective view showing a schematic configuration of aconventional piezoelectric transformer with one output on a secondaryside.

FIG. 15 is a perspective view showing a schematic configuration of aconventional piezoelectric transformer with two outputs on a secondaryside.

FIG. 16 is a block diagram showing an exemplary configuration of aconventional driving circuit for a piezoelectric transformer.

FIG. 17 is a schematic diagram showing a configuration of a cold-cathodetube lighting apparatus in which the piezoelectric transformer shown inFIG. 14 is connected in series to cold-cathode tubes.

FIG. 18 is a schematic diagram showing a configuration of a cold-cathodetube lighting apparatus in which the piezoelectric transformer shown inFIG. 14 is connected in parallel to the cold-cathode tubes.

FIG. 19 is a schematic diagram showing a configuration of a cold-cathodetube lighting apparatus in which the piezoelectric transformer shown inFIG. 15 is connected in series to the cold-cathode tubes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described by way ofillustrative embodiments with reference to the drawings.

Embodiment 1

FIG. 1 is a block diagram showing an exemplary configuration of adriving circuit for a piezoelectric transformer of Embodiment 1according to the present invention. In FIG. 1, reference numeral 1denotes a piezoelectric transformer with two outputs on a secondaryside; 2, 3 denote primary side electrodes of the piezoelectrictransformer 1; 4,5 denote secondary side electrodes of the piezoelectrictransformer 1; 7,8 denote cold-cathode tubes; 10 denotes a drivingfrequency control circuit (DFCC); 11 denotes an input power controlcircuit (IPCC); and 20 denotes a cold-cathode tube output detectorcircuit (ODC).

FIG. 2 is a block diagram showing an exemplary internal configuration ofthe cold-cathode tube output detector circuit 20 in the driving circuitfor a piezoelectric transformer of Embodiment 1 according to the presentinvention. In FIG. 2, the cold-cathode tube output detector circuit 20is composed of resistors 21, 22 and 23, and a differential amplifier 24.

Next, the operation of the driving circuit shown in FIGS. 1 and 2 willbe described.

The input power control circuit 11 supplies an AC power to thepiezoelectric transformer 1 based on a signal of a driving frequency anda voltage level output from the driving frequency control circuit 10. Inaccordance with a ratio between the impedance between the primary sideelectrodes 2 and 3 of the piezoelectric transformer 1 and the impedancebetween the secondary side electrodes 4 and 5 thereof, voltagesstepped-up with respect to the input voltage are output from thesecondary side electrodes 4 and 5. The secondary side electrodes 4 and 5constituting the piezoelectric transformer 1 are formed symmetricallywith respect to the primary side electrodes 2 and 3 formed at the centerof the rectangular plate 6 in the length direction, and the vicinitiesof the secondary side electrodes 4 and 5 are polarized in the samedirection as shown in FIG. 1. Therefore, AC voltages with substantiallythe same amplitude and a phase difference of 180° are output from thetwo secondary side electrodes 4 and 5. The output AC voltages areapplied to the cold-cathode tubes 7 and 8, and currents flowtherethrough in accordance with the respective impedances.

Furthermore, in FIG. 2, the resistor 21 is connected between thecold-cathode tubes 7 and 8, and the electric potential at both ends ofthe resistor 21 is detected by the differential amplifier 24 through theresistors 22 and 23. The driving frequency control circuit 10 and theinput power control circuit 11 control the driving frequency or inputvoltage of the piezoelectric transformer 1 to control the driving of thepiezoelectric transformer 1, based on a signal from the cold-cathodetube output detector circuit 20.

For example, in the case of using cold-cathode tubes of 6W, a currentflowing through one cold-cathode tube is 10 mA, and an applied voltagethereto is 600 V during steady lighting. Before the commencement oflighting, an applied voltage is in a range of 1 kV to 1.5 kV. The outputpower of the piezoelectric transformer 1 is varied depending upon animpedance change of the cold-cathode tubes 7 and 8 connected to thepiezoelectric transformer 1, so that the shapes of the cold-cathodetubes 7 and 8 are designed so as to correspond to required voltage,current and power.

Furthermore, the driving frequency generally is about 40 kHz to 80 kHz.The lower limit of the driving frequency is determined as a value thatis not in an audio range, and the upper limit of the driving frequencyis determined based on the relationship with respect to a system formounting cold-cathode tubes. With a single cold-cathode tube, as thedriving frequency becomes higher, the brightness efficiency isincreased. In the case where an electromagnetic transformer is driven atabout 100 kHz or more, the conversion efficiency is decreased. However,in the case where a piezoelectric transformer is driven at 100 kHz ormore, the conversion efficiency is not decreased. However, a reflectoris placed in parallel between cold-cathode tubes when cold-cathode tubesare mounted on a liquid crystal panel or the like. Therefore, a straycapacitance is generated between the cold-cathode tubes and thereflector. In the case where the driving frequency is high, a currentflows from the cold-cathode tubes to the reflector through the straycapacitance. Then, even if an electric power is supplied from thepiezoelectric transformer, a current flows through the reflector insteadof the cold-cathode tubes, which results in a decrease in a brightnessefficiency with respect to the electric power on the side of thepiezoelectric transformer. Because of this, the upper limit of thedriving frequency is determined.

According to the present embodiment, although one primary side electrode3 of the piezoelectric transformer 1 is grounded, the connection portionbetween the cold-cathode tubes 7 and 8 is not grounded unlike the methodshown in FIG. 19. Therefore, irrespective of whether there are twocold-cathode tubes, the piezoelectric transformer 1 performs the sameoperation as that with respect to one cold-cathode tube. Accordingly,even if the respective impedances of two cold-cathode tubes aredifferent from each other, the difference in brightness is sufficientlysmaller than a recognizable level during steady lighting. Furthermore,the operation in which only one cold-cathode tube lights up at thecommencement of lighting is unlikely to occur.

The present embodiment is not limited to the driving circuit composed ofthe components shown in FIG. 1. Other components may be used as long asthey can function in the same way as in FIG. 1.

Furthermore, it also is possible to set the driving frequency and theinput voltage of the piezoelectric transformer through software by usinga microcomputer, a peripheral apparatus such as a data storage device(e.g., memory) and the like, instead of setting them by the drivingfrequency control circuit 10 and the input power control circuit 11,based on the signal from the cold-cathode tube output detector circuit20.

Embodiment 2

FIG. 3 is a block diagram showing an exemplary configuration of adriving circuit for a piezoelectric transformer of Embodiment 2according to the present invention. In FIG. 3, the same components asthose in FIG. 2 are denoted with the same reference numerals as thosetherein, and the description thereof will be omitted here. In thepresent embodiment, the configuration of the cold-cathode tube outputdetector circuit 20 is different from that of Embodiment 1.

In FIG. 3, the cold-cathode tube output detector circuit 20 of thepresent embodiment is composed of a current transformer 25 and aresistor 26.

Next, the operation of the driving circuit shown in FIG. 3 will bedescribed.

The procedure of controlling the driving of the piezoelectrictransformer 1 is the same as that of Embodiment 1. A primary sidewinding of the current transformer 25 of the cold-cathode tube outputdetector circuit 20 is connected between the cold-cathode tubes 7 and 8.A current induced in a secondary side winding of the current transformer25 is converted to a voltage by the resistor 26 in accordance with acurrent flowing through the primary side winding of the currenttransformer 25. The voltage thus obtained is used as a control signalfor driving control by the driving frequency control circuit 10 and theinput power control circuit 11.

According to the present embodiment, in addition to the effect ofEmbodiment 1, the following effect can be obtained. In the cold-cathodetube output detector circuit 20, a current flowing through thecold-cathode tubes 7 and 8 and a detection signal are separatedelectrically by the current transformer 25. Therefore, a ratio at whicha current flowing through the cold cathode tubes 7 and 8 and a detectionsignal influence each other can be suppressed, and the noise resistanceof the current flowing through the cold-cathode tubes 7 and 8 and thedetection signal can be enhanced.

The present embodiment is not limited to the driving circuit composed ofthe components shown in FIG. 3, and other components may be used as longas they function in the same way as in FIG. 3.

Furthermore, in the same way as in Embodiment 1, it also is possible toset the driving frequency and the input voltage of the piezoelectrictransformer through software by using a microcomputer, a peripheralapparatus such as a data storage device (e.g., memory) and the like,instead of setting them by the driving frequency control circuit 10 andthe input power control circuit 11, based on the signal from thecold-cathode tube output detector circuit 20.

Embodiment 3

FIG. 4 is a block diagram showing an exemplary configuration of adriving circuit for a piezoelectric transformer of Embodiment 3according to the present invention. In FIG. 4, the same components asthose in FIGS. 1, 2, and 3 are denoted with the same reference numeralsas those therein, and the description thereof will be omitted here. Thepresent embodiment is different from Embodiments 1 and 2 in that acontrol signal selection circuit 12 (CSSC) is provided, and the drivingfrequency control circuit 10 and the input power control circuit 11receive a signal detected by the cold-cathode tube output detectorcircuit 20 through the control signal selection circuit 12.

During steady lighting of the cold-cathode tubes 7 and 8, a harmoniccomponent other than a frequency component of an output signal of thepiezoelectric transformer 1 may be superimposed to degrade the qualityof a control signal, depending upon the difference in impedance betweenthe two cold-cathode tubes 7 and 8.

By allowing the control signal selection circuit 12 of the presentembodiment to select or extract only a frequency (driving frequency ofthe piezoelectric transformer 1) component of an output signal of thepiezoelectric transformer 1 among signals of the cold-cathode tubeoutput detector circuit 20, only the frequency component of the outputsignal of the piezoelectric transformer 1 can be used as a controlsignal. Therefore, in addition to the effects of Embodiments 1 and 2,the driving control of the piezoelectric transformer and the brightnesscontrol of the cold-cathode tubes can be performed with a higherprecision.

The present embodiment is not limited to the driving circuit composed ofthe components shown in FIG. 4, and other components may be used as longas they function in the same way as in FIG. 4.

Furthermore, in the same way as in Embodiments 1 and 2, it also ispossible to set the driving frequency and the input voltage of thepiezoelectric transformer through software by using a microcomputer, aperipheral apparatus such as a data storage device (e.g., memory) andthe like, instead of setting them by the driving frequency controlcircuit 10 and the input power control circuit 11, based on the signalfrom the cold-cathode tube output detector circuit 20.

Embodiment 4

FIG. 5 shows, as Embodiment 4 of the present invention, an internalconfiguration in which the driving circuit for a piezoelectrictransformer of Embodiment 1, 2, or 3 is used as an inverter circuit fordriving cold-cathode tubes that function as a backlight of a liquidcrystal panel of a liquid crystal display, a liquid crystal monitor, orthe like

In FIG. 5, reference numeral 30 denotes a liquid crystal panel, 31denotes an inverter circuit, and 7,8 denote cold-cathode tubes.

In a conventional electromagnetic transformer, it is required to keepoutputting a high voltage at the commencement of lighting ofcold-cathode tubes. However, an output voltage of a piezoelectrictransformer is varied in accordance with a load fluctuation at thecommencement of lighting and during lighting of the cold-cathode tubes.Therefore, by using the piezoelectric transformer, an adverse effect onanother circuit system mounted on a liquid crystal panel can beeliminated. Furthermore, an output voltage to the cold-cathode tubes hasa substantially sine wave, so that an unnecessary frequency componentthat does not contribute to lighting of the cold-cathode tubes can bereduced.

Furthermore, the piezoelectric transformer can deal with electric energylarger than that of the electromagnetic transformer per unit volume.Therefore, the piezoelectric transformer can be reduced in volume.Furthermore, the piezoelectric transformer uses vibration in the lengthdirection of a rectangular plate, which is advantageous to thinning ofthe piezoelectric transformer. As a result, the piezoelectrictransformer can be mounted even in a narrow place such as an edge of aliquid crystal panel, which leads to miniaturization and reduced weightof the liquid crystal panel.

Furthermore, in a conventional piezoelectric transformer, a high voltagebased on the number of cold-cathode tubes needs to be generated so as toallow a plurality of cold-cathode tubes to light up. A lighting defectoccurs among a plurality of cold cathode tubes, and the piezoelectrictransformer is driven in an unstable load state. However, according tothe present embodiment, a lighting defect of the cold-cathode tubes isdecreased, and the piezoelectric transformer can be driven stably, whichleads to high reliability and high performance of the liquid crystalpanel itself.

Embodiment 5

FIG. 6 shows, as Embodiment 5 of the present invention, an externalconfiguration in which the liquid crystal panel of Embodiment 4 ismounted on a mobile phone. The liquid crystal panel of Embodiment 4,i.e., the liquid crystal panel 30 with the driving circuit for apiezoelectric transformer of Embodiment 1, 2, or 3 built therein ismounted on, for example, a mobile phone 40 as a device, whereby aplurality of cold-cathode tubes are allowed to light up with onepiezoelectric transformer. Therefore, higher brightness of a liquidcrystal screen can be realized.

Furthermore, by using a piezoelectric transformer instead of anelectromagnetic transformer in order to allow cold-cathode tubes tolight up, electromagnetic noise can be suppressed. Therefore, an adverseeffect on a device due to noise and cross modulation can be eliminated.

In the present embodiment, the case where the liquid crystal panel ofEmbodiment 4 is mounted on a mobile phone has been described. However,the same effect can be obtained even when the liquid crystal panel ismounted on a personal digital assistant, a communication terminal, orthe like.

Embodiment 6

FIG. 7 is a block diagram showing an exemplary configuration of adriving circuit for a piezoelectric transformer of Embodiment 6according to the present invention. In FIG. 7, the same components asthose in FIGS. 1, 2, and 3 are denoted with the same reference numeralsas those therein, and the description thereof will be omitted here. Inthe present embodiment, the configuration of the cold-cathode tubeoutput detector circuit 20 is different from those in Embodiments 1 and2.

In FIG. 7, the cold-cathode tube output detector circuit 20 in thepresent embodiment is composed of a diode 27, an optical isolator(photocoupler) 28, and a resistor 29.

Next, the operation of the driving circuit shown in FIG. 7 will bedescribed.

The procedure of controlling the driving of the piezoelectrictransformer 1 is the same as those of Embodiments 1 and 2. The diode 27and the optical isolator 28 of the cold-cathode tube output detectorcircuit 20 are connected between the cold-cathode tubes 7 and 8. Thediode 27 is connected in parallel to a light-emitting diode built on aninput side of the optical isolator 28 so that a current flows in anopposite direction to that of the light-emitting diode. Light with anintensity in accordance with a current flowing through thelight-emitting diode built in the optical isolator 28 is received by aphototransistor, and a current converted photoelectrically by thephototransistor is converted to a voltage as a detection signal by theresistor 29. The detection signal is used for driving control by thedriving frequency control circuit 10 and the input power control circuit11.

In the present embodiment, in addition to the effect of Embodiment 1, acurrent flowing through the cold-cathode tubes 7 and 8 and a detectionsignal can be separated electrically in a similar manner to that ofEmbodiment 2.

Furthermore, when an inductive element is used for the cold-cathode tubeoutput detector circuit 20, if the difference in impedance between thecold-cathode tubes 7 and 8 is large, or depending upon thecharacteristics of the inductive element, a differential component withrespect to a fluctuation of a current flowing through the cold-cathodetubes 7 and 8 with a change of time may become a noise generation factoron the side of the detection signal.

However, according to the present embodiment, by configuring thecold-cathode tube output detector circuit 20 without using an inductiveelement, a noise generation factor can be eliminated, and a currentflowing through the cold-cathode tubes 7 and 8 and a detection signalcan be separated electrically. Therefore, noise resistance further canbe enhanced.

The present embodiment is not limited to the driving circuit composed ofthe components shown in FIG. 7, and other components may be used as longas they function in the same way as in FIG. 7.

Furthermore, in the same way as in Embodiments 1 and 2, it also ispossible to set the driving frequency and the input voltage of thepiezoelectric transformer through software by using a microcomputer, aperipheral apparatus such as a data storage device (e.g., memory) andthe like, instead of setting them by the driving frequency controlcircuit 10 and the input power control circuit 11, based on the signalfrom the cold-cathode tube output detector circuit 20.

Embodiment 7

FIG. 8 is a block diagram showing an exemplary configuration of adriving circuit for a piezoelectric transformer of Embodiment 7according to the present invention. In FIG. 8, the same components asthose in FIGS. 1, 2, 3 and 7 are denoted with the same referencenumerals as those therein, and the description thereof will be omittedhere. The present embodiment is different from Embodiments 1, 2, and 3in that a control signal selection circuit (CSSC) 12 is provided, andthe driving frequency control circuit 10 and the input power controlcircuit 11 receive a signal detected by the cold-cathode tube outputdetector circuit 20 through the control signal selection circuit 12.

In the same way as in Embodiment 3, by allowing the control signalselection circuit 12 to select or extract only a frequency (drivingfrequency of the piezoelectric transformer 1) component of an outputsignal of the piezoelectric transformer 1 among signals of thecold-cathode tube output detector circuit 20, only a frequency componentof an output signal of the piezoelectric transformer 1 can be used as acontrol signal. Therefore, in addition to the effects of Embodiments 1,2, and 6, the driving control of the piezoelectric transformer and thebrightness control of the cold-cathode tubes can be performed with ahigher precision.

The present embodiment is not limited to the driving circuit composed ofthe components shown in FIG. 8, and other components may be used as longas they function in the same way as in FIG. 8.

Furthermore, in the same way as in Embodiments 1, 2, 3, and 6, it alsois possible to set the driving frequency and the input voltage of thepiezoelectric transformer through software by using a microcomputer, aperipheral apparatus such as a data storage device (e.g., memory) andthe like, instead of setting them by the driving frequency controlcircuit 10 and the input power control circuit 11, based on the signalfrom the cold-cathode tube output detector circuit 20.

Embodiment 8

FIG. 9 is a block diagram showing an exemplary configuration of adriving circuit for a piezoelectric transformer of Embodiment 8according to the present invention. FIG. 10 shows an example in whichthe cold-cathode tube output detector circuit shown in FIG. 9 iscomposed of a diode 27, an optical isolator (photocoupler) 28, and aresistor 29.

In FIGS. 9 and 10, the same components as those in FIGS. 1, 2, 3, 7, and8 are denoted with the same reference numerals as those therein, and thedescription thereof will be omitted here.

In FIGS. 9 and 10, the piezoelectric transformer 1, the drivingfrequency control circuit 10, and the input power control circuit 11 aremounted on a first substrate 32. The cold-cathode tube output detectorcircuit 20 is mounted on a second substrate 33 that is different fromthe first substrate 32. By separating a portion including thepiezoelectric transformer 1 that deals with a relatively large electricpower from a portion including the cold-cathode tube output detectorcircuit 20 that deals with a small electric power by using separatesubstrates, the ability of the cold-cathode tube output detector circuit20 to detect a current flowing through the cold-cathode tubes 7 and 8 isenhanced, and noise (in particular, noise due to the proximity to aground line) can be suppressed from being mixed in a detection signaloutput from the cold-cathode tube output detector circuit 20.

The present embodiment is not limited to the driving circuit composed ofthe components shown in FIGS. 9 and 10, and other components may be usedas long as they function in the same way as in FIGS. 9 and 10.

Furthermore, in the same way as in Embodiments 1, 2, 3, 6, and 7, italso is possible to set the driving frequency and the input voltage ofthe piezoelectric transformer through software by using a microcomputer,a peripheral apparatus such as a data storage device (e.g., memory) andthe like, instead of setting them by the driving frequency controlcircuit 10 and the input power control circuit 11, based on the signalfrom the cold-cathode tube output detector circuit 20.

Embodiment 9

FIG. 11 is a block diagram showing an exemplary configuration of adriving circuit for a piezoelectric transformer of Embodiment 9according to the present invention. FIG. 12 is a view showing an examplein which the cold-cathode tube output detector circuit 20 shown in FIG.11 is composed of a diode 27, an optical isolator (photocoupler) 28, anda resistor 29.

In FIGS. 11 and 12, the same components as those in FIGS. 1, 2, 3, 7, 8,9, and 10 are denoted with the same reference numerals as those therein,and the description thereof will be omitted here.

In FIGS. 11 and 12, the piezoelectric transformer 1, the drivingfrequency control circuit 10, and the input power control circuit 11 aremounted on the first substrate 32. The cold-cathode tube output detectorcircuit 20 and the control signal selection circuit 12 are mounted onthe second substrate 33 that is different from the first substrate 32.By separating a portion including the piezoelectric transformer 1 thatdeals with a relatively large electric power from a portion includingthe cold-cathode tube output detector circuit 20 that deals with a smallelectric power by using separate substrates, the ability of thecold-cathode tube output detector circuit 20 to detect a current flowingthrough the cold-cathode tubes 7 and 8 is enhanced, and noise (inparticular, noise due to the proximity to a ground line) can besuppressed from being mixed in a detection signal output from thecold-cathode tube output detector circuit 20.

In addition, in the same way as in Embodiments 3 and 7, by allowing thecontrol signal selection circuit 12 to select or extract only afrequency (driving frequency of the piezoelectric transformer 1)component of an output signal of the piezoelectric transformer 1 amongsignals of the cold-cathode tube output detector circuit 20, only afrequency component of an output signal of the piezoelectric transformer1 can be used as a control signal. Therefore, in addition to the effectsof Embodiments 1, 2, 6, 7, and 8, the driving control of thepiezoelectric transformer and the brightness control of the cold-cathodetubes can be performed with a higher precision.

The present embodiment is not limited to the driving circuit composed ofthe components shown in FIGS. 11 and 12, and other components may beused as long as they function in the same way as in FIGS. 11 and 12.

Furthermore, in the same way as in Embodiments 1, 2, 3, 6, 7, and 8, italso is possible to set the driving frequency and the input voltage ofthe piezoelectric transformer through software by using a microcomputer,a peripheral apparatus such as a data storage device (e.g., memory) andthe like, instead of setting them by the driving frequency controlcircuit 10 and the input power control circuit 11, based on the signalfrom the cold-cathode tube output detector circuit 20.

Embodiment 10

FIG. 13 shows, as Embodiment 10 of the present invention, an internalconfiguration in the case where the driving circuit for a piezoelectrictransformer of Embodiment 8 or 9 is used as an inverter circuit drivingcold-cathode tubes that function as a backlight of a liquid crystalpanel of a liquid crystal display, a liquid crystal monitor, or thelike.

In FIG. 13, reference numerals 7, 8 denote cold-cathode tubes, 30denotes a liquid crystal panel, 32 denotes a first substrate on whichcircuit elements including a piezoelectric transformer, a drivingfrequency control circuit, and an input power control circuit in aninverter circuit are mounted, and 33 denotes a second substrate on whichcircuit elements including only a cold-cathode tube output detectorcircuit or a cold-cathode tube output detector circuit and a controlsignal selection circuit in an inverter circuit are mounted. The firstsubstrate 32 is placed in proximity to one electrode of each of thecold-cathode tubes 7 and 8, and the second substrate 33 is placed inproximity to the other electrode of each of the respective cold-cathodetubes 7 and 8. The liquid crystal panel of the present embodiment may bemounted on the mobile phone of Embodiment 5 shown in FIG. 6.

In FIG. 13, the same components as those in FIGS. 5, 9, 10, 11, and 12are denoted with the same reference numerals as those therein, and thedescription thereof will be omitted here.

According to the present embodiment, in addition to the effect ofEmbodiment 4, by separating the first substrate 32 that deals with arelatively large electric power including the piezoelectric transformer,the driving frequency control circuit, and the input power controlcircuit from the second substrate 33 that deals with a small electricpower including only the cold-cathode tube output detector circuit orthe cold-cathode tube output detector circuit and the control signalselection circuit, the ability to detect a current flowing through thecold-cathode tubes 7 and 8 is enhanced, and noise (in particular, noisedue to the proximity to a ground line) can be suppressed from beingmixed in a detection signal, in the same way as in Embodiments 8 and 9.

As described above, according to the present invention, even if aplurality of cold-cathode tubes have different impedances, thedifference in brightness during steady lighting is small, andnon-lighting of cold-cathode tubes at the commencement of lighting isunlikely to occur.

Furthermore, according to the present invention, a ratio at which acurrent flowing through the cold-cathode tubes and a detection signalinfluence each other can be suppressed. Therefore, the noise resistanceof a current flowing through the cold-cathode tubes and a detectionsignal can be enhanced.

Furthermore, according to the present invention, only a frequencycomponent of an output signal of the piezoelectric transformer can beused as a control signal. Therefore, the driving control of thepiezoelectric transformer and the brightness control of the cold-cathodetubes can be performed with a higher precision.

Furthermore, a cold-cathode tube light-emitting apparatus, to which thedriving circuit of the present invention is applied so as to control thelight-emission of cold-cathode tubes, is built in a liquid crystalpanel. Because of this, an output voltage of the piezoelectrictransformer is varied in accordance with a load fluctuation at thecommencement of lighting or during lighting of the cold-cathode tubes,so that an adverse effect on another circuit system mounted on a liquidcrystal panel can be eliminated. Furthermore, an output voltage to thecold-cathode tubes has a substantially sine wave. Therefore, anunnecessary frequency component that does not contribute to lighting ofthe cold-cathode tubes can be reduced. Furthermore, the driving circuitof the present invention can be mounted even in a narrow space such asan edge of a liquid crystal panel, which leads to the miniaturizationand reduced weight of the liquid crystal panel.

Furthermore, a cold-cathode tube light-emitting apparatus, to which thedriving circuit of the present invention is applied so as to control thelight-emission of cold-cathode tubes, is built in a liquid crystalpanel. The liquid crystal panel is incorporated into a device such as amobile phone, a personal digital assistant, a communication terminal, orthe like. Because of this, even when the number of cold-cathode tubes issmall, high brightness of a liquid crystal screen can be realized. Inaddition, by using a piezoelectric transformer instead of anelectromagnetic transformer in order to allow the cold-cathode tubes tolight up, electromagnetic noise can be suppressed from being generated,and noise to a device and an adverse effect due to cross modulation canbe eliminated.

The invention may be embodied in other forms without departing from thespirit or essential characteristics thereof. The embodiments disclosedin this application are to be considered in all respects as illustrativeand not limiting. The scope of the invention is indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

1. A cold-cathode tube light-emitting apparatus, comprising: apiezoelectric transformer including a primary side electrode and asecondary side electrode formed on a piezoelectric body and convertingan AC voltage input from the primary side electrode to output theconverted AC voltage from the secondary side electrode; a plurality ofcold-cathode tubes connected to a secondary side of the piezoelectrictransformer; a cold-cathode tube output detector portion connected inseries to the plurality of cold-cathode tubes, for electricallyseparating and detecting a current flowing through the respectivecold-cathode tubes; and a control portion for controlling light-emissionof the plurality of cold-cathode tubes based on a detection signal fromthe cold-cathode tube output detector portion.
 2. The cold-cathode tubelight-emitting apparatus according to claim 1, wherein the piezoelectricbody has a rectangular shape and the secondary side electrode of thepiezoelectric transformer is provided on each end face of thepiezoelectric body in a longitudinal direction, and polarizationdirections of the piezoelectric body in a vicinity of the secondary sideelectrodes are the same in the longitudinal direction.
 3. Thecold-cathode tube light-emitting apparatus according to claim 2, whereinthe number of the cold-cathode tubes is 2n (n is an integer of 1 ormore).
 4. The cold-cathode tube light-emitting apparatus according toclaim 2, wherein the cold-cathode tube output detector portion includesan optical isolator composed of a light-emitting diode and aphototransistor.
 5. The cold-cathode tube light-emitting apparatusaccording to claim 2, wherein the piezoelectric transformer and thecontrol portion are mounted on a first substrate placed in proximity toone electrode of each of the plurality of cold-cathode tubes, and thecold-cathode tube output detector portion is mounted on a secondsubstrate placed in proximity to the other electrode of each of theplurality of cold-cathode tubes.